One common type of electrohydraulic servovalve comprises a first stage torque motor which receives an electrical signal and positions a flapper between a pair of opposed nozzles to control a spool valve and a feedback spring connected to the flapper and to the spool of the spool valve.
Such servovalves normally contain some means of converting an electric input signal to a mechanical output motion. In some designs, the mechanical output motion is very small and can be as small as 20 millionths of an inch. Since repeatability of better than 0.5% is required, it is apparent that the mechanical rigidity of the components which convert electrically generated forces to physical motion must be high. The means for converting the electrical input signal to a mechanical motion is through a device commonly called the torque motor. Application of current to the coils polarizes the armature which reacts with the field in the pole piece air gaps. This results in a moment on the armature and the armature/flapper assembly rotates around the virtual pivot point. Resisting the moment applied to the armature :s the force required to bend the spring tube as a cantilever beam and a pressure unbalance in the two nozzles facing the flapper. The generally accepted methods of attaching the armature to the flapper may be categorized as: clamping, soft soldering, hard soldering and press fitting.
All of these methods provide a metal to metal interface and the necessary rigidity, freedom from friction, stability and long life required by the armature/flapper joint. However, all of the above methods have manufacturing problems which result in added cost, loss of integrity or loss of mechanical or magnetic properties. The ideal attachment method would introduce no undesirable materials such as soldering flux, provide no mechanical stress on the armature to degrade the magnetic properties and not expose the armature/flapper/spring tube assembly to temperatures which may alter the mechanical or magnetic properties of the components.
Among the objectives of the present invention are to provide such a servovalve which overcomes the problems of the prior art; wherein the armature/flapper joint is stress free; wherein the armature and flapper are precisely positioned related to one another; which does not require the use of soldering flux and corrosive problems associated therewith; which has no creep movement under long term stress conditions; which can be readily made in commercial production; and which can be repeatedly and accurately provided in commercial production.
In accordance with the invention, the flapper is connected to the armature of the torque motor by a structural adhesive The joint between the armature and the flapper preferably comprises a one part, heat cured thermosetting structural adhesive.